1. Field of the Invention
The present invention relates to a driving power transmission system for a vehicle and more particularly, to a driving power transmission system capable of varying the transmissive torque depending upon running conditions of the vehicle.
2. Description of the Prior Art
Conventionally, a driving power transmission system for a four-wheel drive vehicle is provided with a torque transmission device capable of changing the transmissive torque depending upon running conditions of the vehicle. The torque transmission device is usually provided with a clutch mechanism for transmitting a driving power, a piston for actuating the clutch mechanism and a pressure generating mechanism for generating a pressure corresponding to a rotational speed difference between an input member and an output member in order to give the piston a moving force toward the clutch mechanism to thereby actuate the clutch mechanism. To make such torque transmission device smaller and lighter, the assignee of the present application proposed an improved torque transmission device in the U.S. patent application Ser. No. 07/165,072, which was filed on Mar. 7, 1988. In the improved torque transmission device, a rotor having plural blades is rotatably disposed within a chamber defined by the housing and the piston, and the chamber is filled with high viscous fluid. When a rotational difference occurs between the input member and the output member, the rotor is rotated within the chamber so that the viscous fluid is compulsorily moved between two side surfaces facing each other with a small distance. With this operation, a pressure proportional to the rotational speed difference is generated within the chamber so that the piston is moved toward the clutch mechanism to bring the clutch mechanism into frictional engagement. The amplitude of the transmissive torque and the response speed of the torque transmission device changes depending on the radial size of the blades. Accordingly, the improved torque transmission device can increase the transmissive torque and the response speed, by increasing the radial size of the blades, so as to allow the vehicle to easily escape from stuck conditions such as muddy places.
However, there is a problem that the radial size of the torque transmission device becomes larger in proportion to the increase of the radial size of the blades, thereby a larger radial space being required for the installation of the torque transmission device within the drive line of the vehicle.
Further, if the transmissive torque is increased by increasing the radial size of the blades, another problem occurs. Namely, if the amplitude of the transmissive torque which is transmitted from the input member to the output member is increased, the amplitude of the transmissive torque which is transmitted back from the output member to the input member is also increased, thereby the rotational torque transmitted from the tires to the drive line being also increased, so that the rotational torque from the tires interferes with a driving torque from the engine and a rotational torque from other tires. This torque interference is called circulation torque and it is produced, for example, when the vehicle turns. The circulation torque causes vibrations of components of the drive line, such as propeller shaft. Therefore, it is required to minimize the circulation torque.